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https://issues.apache.org/jira/browse/CASSANDRA-6601?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13876252#comment-13876252
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Theo Hultberg commented on CASSANDRA-6601:
------------------------------------------
Why does writetime then return null in the case of a null value, and an empty
result when there is no column (as demonstrated above)?
And it's not just writetime, in CQL (I understand that things may be different
with Thrift and compact storage, but this is CQL) selecting a value that is
null is different from selecting a value that does not exist:
Here I create a table, create a column with the value null, and one with a
non-null value. When I ask for the value of the column that explicitly has the
value null, I get a row back with null, just as when I ask for a column that
has a non-null value. However, if I ask for a value that does not exist I get
no rows back. The value null is different from a non-existent column.
{code}
cqlsh:writetime_test> CREATE TABLE foo (pk INT, ck INT, value INT, PRIMARY KEY
(pk, ck));
cqlsh:writetime_test> INSERT INTO foo (pk, ck, value) VALUES (1, 2, null);
cqlsh:writetime_test> INSERT INTO foo (pk, ck, value) VALUES (2, 3, 4);
cqlsh:writetime_test> SELECT value FROM foo WHERE pk = 1 AND ck = 2;
value
-------
null
(1 rows)
cqlsh:writetime_test> SELECT value FROM foo WHERE pk = 2 AND ck = 3;
value
-------
4
(1 rows)
cqlsh:writetime_test> SELECT value FROM foo WHERE pk = 3 AND ck = 4;
(0 rows)
{code}
To further demonstrate the difference I then delete the column where the value
is not null, and after that I get no rows back when I try to get the value:
{code}
cqlsh:writetime_test> DELETE FROM foo WHERE pk = 2 AND ck = 3;
cqlsh:writetime_test> SELECT value FROM foo WHERE pk = 2 AND ck = 3;
(0 rows)
{code}
And then if I update the value to be null, I get a row with null back:
{code}
cqlsh:writetime_test> UPDATE foo SET value = null WHERE pk = 2 AND ck = 3;
cqlsh:writetime_test> SELECT value FROM foo WHERE pk = 2 AND ck = 3;
value
-------
null
(1 rows)
{code}
What am I missing? I know you guys know infinitely more about Cassandra
internals than I do, but you both say null is equivalent to a non-existent
column, and I can't see that. If I can get a result back with a value, even if
it is null, I expect there to be a writetime for that value.
> WRITETIME of a null value does not return a time
> ------------------------------------------------
>
> Key: CASSANDRA-6601
> URL: https://issues.apache.org/jira/browse/CASSANDRA-6601
> Project: Cassandra
> Issue Type: Bug
> Environment: Cassandra 2.0.2
> Reporter: Theo Hultberg
>
> When a cell's value is {{null}} the CQL {{WRITETIME}} function returns null.
> I was expecting it to return a timestamp. Looking at the data in
> {{cassandra-cli}} I can see that the cell has a timestamp.
> Here's a session showing the issue:
> {code}
> cqlsh> CREATE KEYSPACE writetime_test WITH replication = {'class':
> 'SimpleStrategy','replication_factor': '1'};
> cqlsh> USE writetime_test;
> cqlsh:writetime_test> CREATE TABLE writetime_test_table (pk INT, ck INT,
> value INT, PRIMARY KEY (pk, ck));
> cqlsh:writetime_test> INSERT INTO writetime_test_table (pk, ck, value) VALUES
> (1, 2, null);
> cqlsh:writetime_test> SELECT WRITETIME(value) FROM writetime_test_table WHERE
> pk = 1 AND ck = 2;
> writetime(value)
> ------------------
> null
> (1 rows)
> cqlsh:writetime_test> INSERT INTO writetime_test_table (pk, ck, value) VALUES
> (2, 3, 4);
> cqlsh:writetime_test> SELECT WRITETIME(value) FROM writetime_test_table WHERE
> pk = 2 AND ck = 3;
> writetime(value)
> ------------------
> 1389967663822000
> (1 rows)
> cqlsh:writetime_test> SELECT WRITETIME(value) FROM writetime_test_table WHERE
> pk = 3 AND ck = 4;
> (0 rows)
> {code}
> I first insert data where the cell value is {{null}}. When I query its
> writetime I get a row back, but the timestamp is {{null}}.
> I then insert a row with a value, and get a timestamp in the query results,
> as expected.
> Finally I query for the writetime of a cell that does not exist, and get no
> rows back, just to show that there's a difference between calling
> {{WRITETIME}} on {{null}} and on something that doesn't exist.
> Even though the value is {{null}} the cell exists and it has a timestamp. I
> can clearly see the timestamp using {{cassandra-cli}}:
> {code}
> [default@unknown] use writetime_test;
> Authenticated to keyspace: writetime_test
> [default@writetime_test] list writetime_test_table;
> Using default limit of 100
> Using default cell limit of 100
> -------------------
> RowKey: 1
> => (name=2:, value=, timestamp=1389967959822000)
> -------------------
> RowKey: 2
> => (name=3:, value=, timestamp=1389967964749000)
> => (name=3:value, value=00000004, timestamp=1389967964749000)
> {code}
> The reason I want to get the {{WRITETIME}} of a {{null}} value is that I have
> a table where I don't care about the value, I use the column keys as sorted
> set. I still want to know when some of them were written though, but I can't
> ask for the {{WRITETIME}} of something that is part of the primary key, so I
> must use the (otherwise unused) value.
> A workaround is to write some dummy value in the cell, and that's probably
> what I need to do, but this felt like it was a bug, it was at least not what
> I expected.
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